In fluorescence spectroscopy the information available from an experiment is related to the spectral properties of the fluorophore. For example, the anisotropy decay of fluorophores which display nanosecond (ns) decay times can be used to measure motions on the ns timescale. A good number of fluorophores have become available which display red or near infrared (NIR) emission [1-2]. Such probes are widely used in the biochemical and medical applications of fluorescence, including protein labeling, chromatography, measurements in blood, noninvasive medical testing, DNA sequencing and analysis and in vivo measurements [3-13]. Many of the red/NIR fluorophores display high extinction coefficients and good quantum yields, both of which indicate the absorportion and emisson electronic transitions are strongly-allowed. Consequently, the decay times of the red/NIR probes are typically below 4 ns and often below 1 ns, as is predicted by theory [14]. These fluorophores typically display small Stokes' shifts, and scattered light is most difficult to eliminate at wavelengths close to the excitation wavelength.
If slower motions on the μs timescale are of interest then it is necessary to use fluorophores which display μs decay times. Furthermore, intracellular fluorophores which require UV excitation result in a background of undesired emission due to the intrinsic fluorescence of cells and tissues. This autofluorescence from biological samples is mostly on the ns timescale and its intensity decreases at longer excitation and emission wavelengths. The signal-to-background ratio cannot be significantly improved by gated detection after the excitation pulse. Hence, the signals detected with red or NIR probes can be affected by scattered light and/or sample autofluorescence.
For these reasons, for example, there is a need for infrared fluorophores which display long excitation and long emission wavelengths and long decay times and preferably high quantum yields.